applied sciences Article
The Preparation and Characterization of Immobilized TiO2/PEG by Using DSAT as a Support Binder Wan Izhan Nawawi 1, *, Raihan Zaharudin 1,2 , Mohd Azlan Mohd Ishak 1 , Khudzir Ismail 2 and Ahmad Zuliahani 1 1
2
*
Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis, 02600 Arau, Perlis, Malaysia;
[email protected] (R.Z.);
[email protected] (M.A.M.I.);
[email protected] (A.Z.) Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia;
[email protected] Correspondence:
[email protected]; Tel.: +60-4-9882-570; Fax: +60-4-9882-026
Academic Editor: Giorgio Biasiol Received: 30 September 2016; Accepted: 13 December 2016; Published: 23 December 2016
Abstract: Immobilized TiO2 was prepared by adding a small composition of polyethylene glycol (PEG) as a binder, and this paper reported for the very first time the formation of C=O from oxidized PEG, which acted as an electron injector in enhancing photoactivity. Water-based TiO2 with PEG formulation was deposited by using a brush technique onto double-sided adhesive tape (DSAT) as a support binder to increase the adhesiveness of immobilized TiO2 . The photocatalytic activity of immobilized TiO2 -PEG was measured by photodegradation of 12 mg·L−1 methylene blue (MB) dye. The optimum condition of immobilized TiO2 -PEG was observed at TiO2 /PEG-2 (TP2) with 10:0.1 for the TiO2 /PEG ratio, which resulted in a 1.8-times higher photodegradation rate as compared to the suspension mode of pristine TiO2 . The high photodegradation rate was due to the formation of the active C=O bond from oxidized PEG binder in immobilized TiO2 -PEG as observed by Fourier transform infrared and X-ray photoelectron spectroscopy analyses. The presence of C=O has escalated the photoactivity by forming an electron injector to a conduction band of TiO2 as proven by higher photoluminescence intensity obtained for TP2 as compared to pristine TiO2 . The TP2 sample has also increased its adhesiveness when DSAT is applied as a support binder where it can be recycled up to eight times and comparable to recent photocatalysis cycle developments. Keywords: immobilization; titanium dioxide; oxidized PEG; support binder; methylene blue
1. Introduction Titanium oxide (TiO2 ) is a semiconductor that is widely known as a photocatalyst for the photodegradation of organic pollutants. According to Karimi et al. [1], when TiO2 is illuminated by a light with energy higher than its band gap energy, electron–hole pairs diffuse out, creating negative electrons and oxygen that combine to become O2 − , while the positive electric holes and water generate hydroxyl radicals. This highly active oxygen species can then oxidize organic pollutants. For over three decades, modifications on TiO2 have improved two main issues. The first is to increase catalytic reaction performance where photocatalytic improvement of TiO2 has been studied by many researchers. This is important in order to make the photocatalyst become active in the wide solar spectrum since the TiO2 semiconductor is only active under high energy ultraviolet (UV) light [2]. Basically, there are two common types of modifications used in preparing the visible light TiO2 , which are the modification of bulk and the surface of TiO2 . Bulk modification often resulted in the narrowing of band gap energy, whereas surface modification does not change the band gap energy. However, surface modification successfully activates the photocatalyst under visible light by accepting
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the electron from the sensitizing agent. The second is a recovery process of this photocatalyst after being treated with organic pollutants where the photodegradation of organic pollutants using TiO2 is basically applied under suspension mode, which provides a high surface to volume ratio [3]. However, the recovery process of this suspension of TiO2 powder with treated wastewater requires filtration. Separation of this nano-sized TiO2 will clog the filter membrane and eventually penetrate through the porous filter, making the recovery process less effective [4]. Immobilization of TiO2 has gained great interest since it can easily separate TiO2 with treated wastewater [5]. Many papers with different types of polymers in immobilized TiO2 have reported these findings after the first technique was discovered by Tennakone et al. [6]. Polymer is normally added as a binder in immobilized TiO2 to improve its durability, temperature resisting ability and absorbance affinity towards pollutants [7–9], and some polymers are also able to increase TiO2 photoactivity [10–25]. In most cases, prepared immobilized TiO2 used solvent-based polymers, such as polythene sheets [10], thin polythene films [11], polystyrene (PS) beads [12], expanded polystyrene (EPS) beads [13], cellulose microspheres [14], fluoro polymer resins [15], polyethylene terephthalate (PET) bottles [16], polypropylene (PP) granules [17], cellulose fibers [18], polypropylene fabric (PPF) [19], polyvinyl chloride (PVC) [20], polycarbonate (PC), poly(methyl methacrylate) (PMMA) [21], polyvinyl acetate (PVAc) [22], poly(styrene)-co-poly(4-vinylpyridine) (PSP4VP) [23], rubber latex (an elastic hydrocarbon polymer) [24], parylene and tedlar [25]. Recently, water-based polymer binders have shown a vast potential of usage in immobilized TiO2 for commercialization because they are environmentally friendly and economic. Water-based polymers, such as polyvinyl alcohol (PVA), polyaniline (PANI), polyvinyl pyrolydone (PVP) and polyethylene glycol (PEG), have performed greatly in water treatment separation, which also resulted in a significant photosensitivity to the visible light region [26–28]. For instance, Lei et al. [29] decorated TiO2 /PVA particles through a heat treatment method and obtained high photoactivity since the Ti–O–C bond generated led the immobilized sample to become fully in contact with the organic pollutant. TiO2 /PVA developed by Yang et al. [30] showed notable photoactivity under visible light irradiation due to the presence of conjugated polymer. They found that the conjugated molecules adsorbed on the TiO2 surface and excited the electrons, thus injecting the electrons into the conduction band using visible light. PANI in immobilized TiO2 studied by Nawi et al. [31] has shown a significant photocatalytic improvement by acting as a hole scavenger in TiO2 under photodegradation of reactive red 4 (RR4) dye. PEG is a polymer that is able to form a uniform surface, smooth coating and well-defined size particles [32]. PEG is a hydrophilic polymer, which is suitable for coating, as it reduces the formation of a cracked surface in the immobilized system [33]. Most of the reports on immobilized TiO2 with PEG were identified to enhance photoactivity due to the effect of porosity and larger specific surface area [34,35]. Trapalis et al. [36] found that the porosity increased with PEG amount introduced in the film. However, no detailed study was conducted on surface chemical interaction between PEG and TiO2 photocatalyst since the oxidation will only take place in the photocatalysis process. Technically, TiO2 and polymer mixed ratios are a very critical factor. Excessive binder makes immobilized film strong, but reduces its photoactivity; however, a low amount of polymer makes TiO2 leach out easily. This explained the lack of data discussed by other researchers on the surface chemical interaction of PEG in immobilized TiO2 . According to Wang et al. [37], visible light photocatalytic activity could also be enhanced by carbon-sensitized TiO2 , in which the carbon could be originated from titanium alkoxide and mainly existed as the C–C bonds (carbonaceous species), C=O and O=C–O bonds (carbonate species). Since visible light active TiO2 using surface modification cannot be judged by using UV–Vis diffuse reflectance spectra (DRS), the easiest way to detect the characteristic of the visible light activity of this modified TiO2 is through photoluminescence (PL) analysis where the highest PL intensity represented the highest photoactivity of the sample. Previously, we have discovered the ability of double-sided adhesive tape (DSAT) to become a support binder in immobilized TiO2 -only [38]. DSAT has greatly improved immobilized TiO2 -only in
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its strength and recyclability. Photocatalytic activity of this immobilized TiO2 -only, however, is slightly lower as compared to TiO2 in suspension mode [39]. In this study, immobilized TiO2 mixed with a small amount of PEG 6000 was prepared using DSAT as a support binder to observe the interaction between TiO2 and PEG during photocatalysis process. The photoactivity of this immobilized TiO2 -PEG was observed by photodegradation of methylene blue, applied with specific parameters. 2. Results and Discussion 2.1. Characterization of Immobilized TiO2 A method for immobilizing TiO2 using DSAT was thoroughly described in the previous article [40]. In this work, a similar approach was taken where TiO2 was coated onto DSAT as a support binder, but in the presence of a specific amount of PEG. Table 1 shows the experimental condition and pseudo first order rate constant of immobilized TiO2 and the pristine TiO2 sample in degrading methylene blue (MB) dye. As can be seen in Table 1, the increased amount of PEG in the formulation solutions has shown a significant increase of PEG in immobilized TiO2 -PEG samples detected by FTIR analysis. The 1160- and 2900-nm peaks of C–O and C–H stretching, respectively, as shown in Figure 1, corresponded to the PEG peaks based on similar peaks presented on the FTIR spectrum for pure PEG studied by Hyma et al. [41]. Photocatalytic activity of washed immobilized TiO2 -PEG was increased by increasing the amount of PEG from 0.05 to 0.1 g based on its photodegradation rate values. Photocatalytic activity starts to decrease beyond 0.1 g of PEG due to the agglomeration of PEG in TiO2 particles, which makes it harder for a large amount of light to penetrate through the surface particles of TiO2 . Mukherjee et al. [39] have reported same behavior on immobilized TiO2 with PVA. The catalyst loading effect of immobilized TiO2 with 0.1 g of PEG samples has shown a different photocatalytic activity. Increasing the amount of catalyst loading from 0.05, 0.1, 0.2 and 0.3 grams will significantly increase the photodegradation rates from 0.039, 0.048, 0.048 and 0.087 min−1 , respectively. The high photodegradation rate at 0.3 g was due to the high adsorption capacity of the photocatalyst thin film composite that enhanced the decolorization process of MB dye, and eventually, this dye pollutant becomes degraded by the photocatalysis process. However, too much adsorption capacity beyond 0.3 g of catalyst loading for the immobilized TiO2 -PEG sample makes the photodegradation rate become 2.5-times slower. This observation was due to the excessive adsorbed MB dye on the surface of immobilized TiO2 -PEG, thus forming a thick layer coating on the TiO2 surface and reducing the penetration of light for photocatalysis. It can be deduced that the immobilized TiO2 -PEG ratio of 10:0.1 at a 0.3-g catalyst loading is the optimum immobilized TiO2 -PEG with a photodegradation rate 1.8-times faster compared to pristine TiO2 ; an optimum sample named as TiO2 /PEG-2 (TP2) (Table 1). Table 1. The experimental condition and pseudo first order rate constant of immobilized TiO2 and the control sample in degrading methylene blue (MB) dye. TiO2 Sample
Loading (g)
Pristine TiO2 TP0 TP1 TP2 TP3 TP4 TP5 TP6 TP7 TP8 a
0.3 0.3 0.3 0.3 0.3 0.3 0.05 0.1 0.2 0.4
Mode (S a or I b )
Amount of PEG (wt %)
S I I I I I I I I I
0.00 0.00 0.05 0.10 0.15 0.20 0.10 0.10 0.10 0.10
Ratio (TiO2 /PEG) 10:0 10:0 10:0.05 10:0.1 10:0.15 10:0.2 10:0.1 10:0.1 10:0.1 10:0.1
Rate Constant k (min−1 )
SBET (m2 ·g−1 )
Washed
Unwashed
50.00 49.25 88.30 -
0.048 0.054 0.080 0.087 0.081 0.040 0.039 0.048 0.048 0.030
0.048 0.041 0.017 0.024 0.030 0.039 0.048 0.028 0.015
Suspension; b immobilize. SBET : surface area; TP0 to TP8, TiO2 /PEG-0 to TiO2 -PEG-8.
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Transmittance Transmittance %%
4 of 16 4 of 16
120 120
60 60
C-H C-H
4000 4000
TP1 (0.5% PEG) TP1 (0.5% PEG) TP2 (1.0% PEG) TP2 (1.0% PEG) TP3 (1.5% PEG) TP3 (1.5% PEG) TP4 (2.0% PEG) TP4 (2.0% PEG)
3000 3000 3000 3000
C-O C-O
2000 2000 2000 2000
1000 1000 1000 1000
Wavelength(cm (cm-1-1) ) Wavelength Figure 1.1. FTIR FTIR spectra spectra ofof different different percentages percentages ofof polyethylene polyethylene glycol (PEG) (PEG) inin immobilized immobilized Figure Figure 1. FTIR spectra of different percentages of polyethylene glycol glycol (PEG) in immobilized TiO2 -PEG. TiO2-PEG. 2-PEG. TiO
2.2.2.2. XRD Analysis XRD Analysis 2.2. XRD Analysis Figure 2 shows TiO TP0 (immobilized (immobilizedTiO TiO DSAT) and TP2 Figure showsthe theXRD XRDpatterns patternsfor forpristine pristine TiO TiO22,2,,TP0 TP0 2 2DSAT) and TP2 Figure 22 shows the XRD patterns for pristine (immobilized TiO 2 DSAT) and TP2 (immobilized TiO DSAT)samples. samples.AllAll peaks detected as anatase andphases. rutile There phases. 2 /PEG (immobilized TiO 2/PEG DSAT) peaks werewere detected as anatase and rutile (immobilized TiO 2/PEG DSAT) samples. All peaks were detected as anatase and rutile phases. There There is no phase transformation occurring in TP0 and TP2, since the immobilization processes nophase phasetransformation transformationoccurring occurringin inTP0 TP0and andTP2, TP2,since sincethe theimmobilization immobilizationprocesses processesof ofTP0 TP0and and isisno of TP2 TP0were and prepared TP2 were prepared under low temperature (120 ◦ C). Phase transformation in the under low temperature (120 °C). Phase transformation in the presence of organic TP2 were prepared under low temperature (120 °C). Phase transformation in the presence of organic presence of organic binder observed when the calcining temperature happened C [42]. binderwas was observed whenwas thecalcining calciningtemperature temperature happened 900°C °C[42]. [42]. Wangetetat al.900 [43]◦also also binder observed when the happened atat900 Wang al. [43] Wang et al. [43] also found that the phase transformation TiO calcined without organic binder found that the phase transformation for TiO 2 calcined for without organic binder occurred at found that the phase transformation for TiO2 calcined without2 organic binder occurred at aa temperature of700 700°C °Conwards. onwards. All peaksfor forAll allsamples samples correspond tocorrespond thecharacteristic characteristic peakof ofthe the occurred at a temperature of 700 ◦All C onwards. peaks forcorrespond all samples to thepeak characteristic temperature of peaks all to the ◦ ◦ anatase and rutileand phases ofTiO TiO2 2nanoparticles nanoparticles detectedatat2θ 2θfrom from15° 15° to65° 65°by by15 using lowby angle peak of the anatase rutile phases of TiO2 nanoparticles detected at to 2θ from to 65 using anatase and rutile phases of detected using low angle XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction peaks low angle XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction peaks displayed the good good crystallinity of the the prepared nanoparticles [44]. Pristine TiO 2 has thethe highest peaks displayed the good crystallinity of the prepared nanoparticles [44]. Pristine TiO highest displayed the crystallinity of prepared nanoparticles [44]. Pristine TiO 2 has the highest 2 has crystallinity as compared to TP0 and TP2, where TP2 is the lowest. This is due to the presence of crystallinity asas compared to to TP0 and TP2, where TP2 is the lowest. This is due to the presence of DSAT crystallinity compared TP0 and TP2, where TP2 is the lowest. This is due to the presence of DSAT and DSAT + PEG in TP0 and TP2, respectively. The increased porosity in both immobilized and DSAT + DSAT PEG in+ TP0 TP2,and respectively. The increased porosity in bothinimmobilized samples DSAT and PEGand in TP0 TP2, respectively. The increased porosity both immobilized samples is the main factor for low crystallinity and broad peaks, as reported by Kim [45]. In short, no the main for low crystallinity andpeaks, broad peaks, as reported by [45]. Kim [45]. In short, no is samples the mainisfactor forfactor low crystallinity and broad as reported by Kim In short, no phase phase transformations occurred for any of the samples prepared under low heat temperature. phase transformations occurred any samples of the samples prepared low temperature. heat temperature. transformations occurred for anyfor of the prepared underunder low heat
Intensity Intensity(a.u) (a.u)
5000 5000 4000 4000 4000 4000 3000 3000
A A
TP2 TP2
R R
A A R A R A
R R
RA RA
A A
TP0 TP0
2000 2000 2000 2000
A – anatase A – anatase R – rutile R – rutile
1000 1000 0000
Pristine TiO Pristine TiO2 2 15 15 15 15
30 30 30 30
theta 22theta
45 45 45 45
60 60 60 60
Figure2.2.XRD XRDpatterns patternsofofpristine pristine TiO2,2,TP0 TP0 andTP2 TP2samples. samples. Figure Figure 2. XRD patterns of pristineTiO TiO2 , TP0and and TP2 samples.
A A
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2.3. SEM Images Images 2.3. SEM Figure the the illustrative SEM images of the TP0 aftersurfaces the photocatalytic Figure 3a,b 3a,bshows shows illustrative SEM images of and the TP2 TP0surfaces and TP2 after the degradation process. Few pores were observed in TP0, while for TP2, a variety of porous photocatalytic degradation process. Few pores were observed in TP0, while for TP2, a structures variety of evolved on the surface. As on shown in FigureAs 3b, the pore depth is around theisPEG concentration porous structures evolved the surface. shown in Figure 3b,larger the pore depth larger around the of 8.0 g/100 mL as compared to TP0. It can be concluded that the porous structure of TP2 thin films is PEG concentration of 8.0 g/100 mL as compared to TP0. It can be concluded that the porous structure related to the molecular weight of PEG. The increased concentration of PEG has granted the formation of TP2 thin films is related to the molecular weight of PEG. The increased concentration of PEG has of big pores. It is obvious thatpores. the catalyst morphologies inmorphologies Figure 3b are highly to granted the formation of big It is obvious that theshown catalyst shown presumed in Figure 3b perform an optimum photocatalytic activity. In this regard, Bing et al. [46] stated that the increased are highly presumed to perform an optimum photocatalytic activity. In this regard, Bing et al. [46] porosity in the the increased immobilized TiO2 /PEG is accountable for the increased photodegradation of the stated that porosity in thefilm immobilized TiO2/PEG film is accountable for the increased methyl orange model pollutant photodegradation of the methyldye. orange model pollutant dye.
Figure 3.3.Scanning Scanning electron micrograph of morphologies surface morphologies for and the(b)(a) and Figure electron micrograph of surface for the (a) TP0 TP2TP0 samples. (b) TP2 samples. EHT, extra high tension. EHT, extra high tension.
2.4. N2 Adsorption-Desorption 2.4. N2 Adsorption-Desorption The porous structure of TP2 is displayed by N2 adsorption-desorption measurement as The porous structure of TP2 is displayed by N2 adsorption-desorption measurement as presented. presented. It was found that the TP2 sample in Figure 4 exhibited the type IV nitrogen isotherm It was found that the TP2 sample in Figure 4 exhibited the type IV nitrogen isotherm according to according to the International Union of Pure and Applied Chemists (IUPAC) classification; thus, this the International Union of Pure and Applied Chemists (IUPAC) classification; thus, this indicated indicated that the TP2 sample is a mesoporous structure. According to Li et al. [47], a large surface that the TP2 sample is a mesoporous structure. According to Li et al. [47], a large surface area with a area with a mesoporous structure is favorable to obtain a high photocatalytic activity, as it promotes mesoporous structure is favorable to obtain a high photocatalytic activity, as it promotes adsorption, adsorption, desorption and diffusion of reactants and products. The BET surface area of TP2 was desorption and diffusion of reactants and products. The BET surface area of TP2 was enhanced enhanced to 88.3 m2·g−1 as compared to pristine TiO2, which was circa 50 m2·g−1, where a 43.4% to 88.3 m2 ·g−1 as compared to pristine TiO2 , which was circa 50 m2 ·g−1 , where a 43.4% increment increment occurred due to the increased porosity and number of pores. By comparing TP0 with TP2, occurred due to the increased porosity and number of pores. By comparing TP0 with TP2, it is clear that the BET surface area for TP2 was increased up to 44% as compared to TP0, which was it is clear that the BET surface area for TP2 was increased up to 44% as compared to TP0, circa 49.25 m2·g−1. The results of TP2 surface area measurements are consistent with the results of which was circa 49.25 m2 ·g−1 . The results of TP2 surface area measurements are consistent photocatalytic activity where the TP2 sample gave a higher photocatalytic degradation rate of 0.087 with the results of photocatalytic activity where the TP2 sample gave a higher photocatalytic min−1 as compared to pristine TiO2, which is 0.048 and 0.054 min−1 for TP0, respectively. A higher degradation rate of 0.087 min−1 as compared to pristine TiO2 , which is 0.048 and 0.054 min−1 for TP0, BET surface area is vital for the adsorption and desorption of dyes and catalysts, since it encourages respectively. A higher BET surface area is vital for the adsorption and desorption of dyes and catalysts, higher photocatalytic performance. In addition, PEG had successfully enhanced the possibility of the since it encourages higher photocatalytic performance. In addition, PEG had successfully enhanced pollutant being trapped within the pores and showed better catalytic activity by providing the possibility of the pollutant being trapped within the pores and showed better catalytic activity by additional surface active groups [48]. providing additional surface active groups [48].
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Quantity Adsorbed (mmol Quantity Adsorbed (mmol g-1)g-1)
20 20 20 1515 1515
desorption desorption adsorption adsorption
1010 1010 5
5 5 5 00 0 000 0 0
0.2 0.2 0.2 0.2
0.4 0.4 0.4 0.4
0.6 0.6 0.6 0.6
0.8 0.8 0.8 0.8
1.0 1.0 1.0 1.0
Relative Pressure (P/P0) Relative Pressure (P/P0)
Figure 4. N2 adsorption–desorption isotherms of the washed immobilized TiO2/PEG (TP2). Figure (TP2). Figure4.4. N N22 adsorption–desorption adsorption–desorption isotherms isotherms of of the the washed washed immobilized immobilized TiO TiO22/PEG /PEG (TP2).
2.5. FTIR 2.5. FTIR FTIR was used to acquire advanced information of the chemical bonding in TP0, washed TP2 was used used toacquire acquire advanced information of chemical bonding indifferences, TP0, washed TP2 and unwashed TP2 to films, as shown in Figure 5. Theofspectra showed significant which FTIR was advanced information thethe chemical bonding in TP0, washed TP2 and −1. All spectra and unwashed TP2 films, as shown in 5. The spectra showed significant which were due to rising bands from theFigure of PEG at 3389.77 and 3375.15differences, cm unwashed TP2the films, as shown in Figure 5.addition The spectra showed significant differences, which were due −1. All spectra − 1 . All were tobands the rising bands from the addition ofofPEG 3389.77 3375.15 showed broad bands, which indicated presence O–H stretching vibrations ofcm water absorbed to the due rising from the addition ofthe PEG at 3389.77 andat3375.15 cmand spectra showed broad showed broad bands, which indicated the presence ofvibrations O–H vibrations of water with the hydroxyl group the photocatalyst surface. Thestretching O–H bond was also noticed in both bands, which indicated the on presence of O–H stretching of water absorbed with the absorbed hydroxyl with the group on the surface. The O–H bond was also noticed in both spectra at hydroxyl 1637.32 and 1637.45 cm−1photocatalyst . The 2900 cm−1also wasnoticed detected all spectra corresponding group on the photocatalyst surface. Thepeak O–Hatbond was in in both at 1637.32 and −1 −1 − 1 − 1 spectra at 1637.32 at 2900 was detected in all spectra to C–H bonds. 1637.45 cm . Theand peak1637.45 at 2900cm cm . The waspeak detected in cm all spectra corresponding to C–Hcorresponding bonds. to C–H bonds.
160
Transmittance Transmittance %%
160
120 120
80 80
40 40
4000 4000
TP0 TP2 TP0 Unwashed TP2 TP2 DSAT Unwashed TP2 DSAT
C-H C-H
3000 3000 3000 3000
C-C C-C 2000 2000 2000 2000 -1
Wavelength (cm ) Wavelength (cm-1)
C=O C=O
C-O C-O 1000 1000
Figure FTIR spectra spectra of of TP0, Figure 5. 5. FTIR TP0, TP2 TP2 (washed (washed TP2), TP2), unwashed unwashed TP2 TP2 and and double-sided double-sided adhesive adhesive tape tape (DSAT) samples. Figure 5.samples. FTIR spectra of TP0, TP2 (washed TP2), unwashed TP2 and double-sided adhesive tape (DSAT) (DSAT) samples.
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Interestingly, strong absorption absorption in in the the region region of of 1705.02 1705.02 cm cm−−11,, Interestingly, washed washed TP2 TP2 spectra spectra showed showed aa strong which thethe other hand, a new peak detected at which justified justified that that the the carbonate carbonatespecies speciesC=O C=Oexisted. existed.OnOn other hand, a new peak detected −1, which − 1 1160.06 cm is assigned to C–O species, was also present in the spectra. The presence of C=O at 1160.06 cm , which is assigned to C–O species, was also present in the spectra. The presence affirmed that TiO 2 has strongly with PEG, which could potentially lead to the high of C=O affirmed that TiOreacted 2 has reacted strongly with PEG, which could potentially lead to the high photoactivity of TP2. TP2. The TheC=O C=Obonds bondsdetected detectedwere weredue due oxidation of PEG in irradiated the irradiated photoactivity of to to thethe oxidation of PEG in the TP2 TP2 film. As shown in Figure 5, the carbonaceous species C–C bond was detected in all samples film. As shown in Figure 5, the carbonaceous species C–C bond was detected in all samples except TP0. except TP0.the Generally, the IR forproduced DSAT also the but C=Othis bonds, but this can Generally, IR spectrum forspectrum DSAT also theproduced C=O bonds, factor can be factor discarded be discarded because during FTIR analysis of all samples, each sample’s powder was carefully because during FTIR analysis of all samples, each sample’s powder was carefully scratched and taken scratched andexcluding taken for the analysis, the DSAT. This is by further supported IR spectrum for analysis, DSAT.excluding This is further supported the IR spectrumby ofthe unwashed TP2, of unwashed TP2, where the formation C=O bonds was5.absent in Figure 5. In brief, though where the formation of C=O bonds wasofabsent in Figure In brief, even though the even composition the structure of the TP2 film do notthe greatly change, the photoactivity andcomposition structure of and the investigated TP2investigated film do not greatly change, photoactivity efficiency of TP2 efficiency TP2 in Tableattributed 1 was indirectly attributedof toC=O the formation C=O bond that promotes in Table 1 of was indirectly to the formation bond thatof promotes more reactions to more reactions to take place. take place. 2.6. X-ray X-ray Photoelectron Photoelectron Spectroscopy Spectroscopy (XPS) (XPS) 2.6. XPS is is used used to determine determine the the binding binding energy energy of elements elements detected detected in in unwashed unwashed and and washed washed XPS immobilized TP2 TP2 samples samples in in order order to examine the effect of washing. This This effect effect had had successfully successfully immobilized produced an an oxidized oxidized PEG. PEG. The The O1s O1s and and C1s C1s spectra spectra of of TP2 TP2 unwashed unwashed are are shown shown in in Figure Figure 6a,b, 6a,b, produced while Figure 6c,d represents the O1s and C1s spectra for washed TP2 sample.
Figure C1s X-ray photoelectron spectroscopy (XPS) spectrums of immobilized for Figure 6. 6. The TheO1s O1sand and C1s X-ray photoelectron spectroscopy (XPS) spectrums of immobilized (a,b) unwashed TP2TP2 andand (c,d)(c,d) washed TP2.TP2. CPS,CPS, cycles per per second. for (a,b) unwashed washed cycles second.
All All samples samples demonstrated demonstrated the the chemical chemical composition compositioninformation informationwhere wherethree threeelements elementsexisted: existed: Ti, C and O. Figure 6a represents the O1s characteristic peaks of Ti–O and C–O at 529.9 Ti, C and O. Figure 6a represents the O1s characteristic peaks of Ti–O and C–O at 529.9and and531.6 531.6eV, eV, respectively, respectively, while while C–O C–O and and C–C C–C bonds bondsare are given givenby bythe thevalues valuesof of288.2 288.2and and284.8 284.8eV, eV,respectively, respectively, in Figure 6b. The O1s spectra in Figure 6c of washed TP2 revealed peaks at 529.4, 531.0 and 532.9 eV attributed to Ti–O, C–O and C=O bonds, respectively. Figure 6d shows the spectrum of washed TP2
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in Figure 6b. The O1s spectra in Figure 6c of washed TP2 revealed peaks at 529.4, 531.0 and 532.9 eV attributed to Ti–O, C–O and C=O bonds, respectively. Figure 6d shows the spectrum of washed TP2 in theAppl. C1sSci.spectra deconvoluted into three distinct curves, which are represented by three8 forms of 2016, 6, 451 of 16 carbon as C–C, C–O and C=O at 284.8, 286.7 and 288.7 eV, respectively. As such, the formation of C=O theonly C1s be spectra deconvoluted which arewas represented by three forms of TP2. bondin can found in washed into TP2.three Yet, distinct no peakcurves, for C=O bond presented in unwashed carbon as C–C, C–O and C=O at 284.8, 286.7 and 288.7 eV, respectively. As such, the formation of This occurrence of C=O bond could be due to the oxidation process of hydroxyl radicals with PEG, C=O bond can only be found in washed TP2. Yet, no peak for C=O bond was presented in unwashed introduced by the washed sample of TP2 in Figure 6c,d. TP2. This occurrence of C=O bond could be due to the oxidation process of hydroxyl radicals with PEG, introduced by the washed sample of TP2 in Figure 6c,d.
2.7. UV–Vis DRS and Visible Light Photodegradation Studies
2.7. UV–Vis DRS reflectance and Visible Light Photodegradation Studies UV–Vis diffuse spectra (UV–Vis DRS) of pristine TiO2 , unwashed and washed TP2 are shown in UV–Vis Figure 7a. All samples have a bit of difference the pattern in UV–Visand diffuse reflectance diffuse reflectance spectra (UV–Vis DRS) ofin pristine TiO2, unwashed washed TP2 spectra. It is obvious that7a. pristine TiO2 has highest absorbance UV–Visinspectra are shown in Figure All samples havethe a bit of difference in theinpattern UV–Visfollowed diffuse by reflectance It is TP2 obvious that pristine TiO2 has highest TiO absorbance UV–Vis washed TP2 andspectra. unwashed samples. Even though thethe modified has lower spectra absorbance 2 or TP2in followed to byunmodified washed TP2 pristine and unwashed TP2 result samples. Even though the modified TiO 2 or TP2property has as compared TiO2 , this is expected because there are no bulk lower absorbance as compared to unmodified pristine TiO 2, this result is expected because there are modifications other than on the TP2 surface. Therefore, the chemical properties for TP2 and its band no bulk property modifications other than on the TP2 surface. Therefore, the chemical properties for gap energy do not notably change even after surface alteration. According to Marcela et al. [49], TP2 and its band gap energy do not notably change even after surface alteration. According to from the solid state band theory, the absorption coefficient can be described as a function of incident Marcela et al. [49], 2from the solid state band theory, the absorption coefficient can be described as a −1 ), A is a constant, photon energy; (αhν) photon = A(hνenergy; − Eg ),(αhν) where is the (cmcoefficient 2 = α function of incident A(hν − Egabsorption ), where α iscoefficient the absorption (cm−1), hν (eV) is the energy of excitation and E is the band gap energy. The band gap energy can be A is a constant, hν (eV) is the energy of gexcitation and Eg is the band gap energy. The band gap 2 2 vs. plot performed (αhν)byvs. hν. The Tauc on Tauc the photon energy-axis gives thegives value energyby canplotting be performed plotting (αhν) hν. The plot on the photon energy-axis theof the directvalue bandofgap of semiconductors [50]. the energy direct band gap energy of semiconductors [50].
a)
TiO2 TP2- Washed TP2- Unwashed
b)
(αhν)2
Abs
0.60.6
0.40.4
TiO2 TP2- Washed TP2- Unwashed 1.51.5
0.2 0.2 0.50.5
300 300
400 400
33
500 500
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c)100
d)100 TP0 TP2 TP2 unwashed
TP0 TP2 TP2 unwashed
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55
60 40 20
50
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0 0
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Figure 7. Graph plotsfor: for: (a) (a) UV–Vis UV–Vis diffuse diffuse reflectance (DRS); (b) (b) Tauc’s equation; Figure 7. Graph plots reflectancespectra spectra (DRS); Tauc’s equation; (c) percentage remainingofofMB MB dye lightlight irradiation; and (d)and adsorption study. Abs,study. (c) percentage remaining dye under undervisible visible irradiation; (d) adsorption absorption. Abs, absorption.
Figure 7b shows a graph of (αhν)2 vs. hν, named as Tauc’s graph plot. Theoretically, it seems
2 vs. hν, named as Tauc’s graph plot. Theoretically, it seems Figure shows graphfor of all (αhν) that the7b band gap aenergy samples did not change and stayed approximately at 3.1 eV. As that the band for allofsamples did not change and stayed approximately at light 3.1 eV. As such, such, the gap bandenergy gap energy TiO2 photocatalyst in TP0 did not change to a visible active
response, and this was similarly observed in Figure 7c where no photodegradation of 12 mg·L−1 MB
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the band gap energy of TiO2 photocatalyst in TP0 did not change to a visible light active response, and this was similarly observed in Figure 7c where no photodegradation of 12 mg·L−1 MB was Appl. Sci. 451 16 observed for2016, TP06, under visible light irradiation. Only decolorization of MB was observed as9 of reported on the adsorption site of the porous surface of TP0 as proven by the adsorption graph in Figure 7d was observed for TP0 under visible light irradiation. Only decolorization of MB was observed as and discussed in our previous study [51]. Unwashed TP2 showed a low photocatalytic degradation of reported on the adsorption site of the porous surface of TP0 as proven by the adsorption graph in MB dye due7dtoand thediscussed C–C bond that makes TP2Unwashed become slightly activea under the visible light Figure in in ourPEG previous study [51]. TP2 showed low photocatalytic condition. Surprisingly, TP2 shows an active photodegradation under visible light where a complete degradation of MB dye due to the C–C bond in PEG that makes TP2 become slightly active under the − 1 decolorization 12 mg·L Surprisingly, MB was achieved at 75anmin. Although there is nounder shifting occur in band visible lightofcondition. TP2 shows active photodegradation visible light where aofcomplete of 12modifications, mg·L−1 MB wasit achieved at that 75 min. there is noability gap energy TP2 due decolorization to its sole-surface is believed the Although visible light active shifting in formation band gap energy of TP2 due tofrom its sole-surface modifications, it is believed the by in TP2 is dueoccur to the of C=O resulting the oxidation of C–O bond in PEG that detected visible light active ability in TP2 is due to the formation of C=O resulting from the oxidation of C–O XPS and FTIR spectrums, which have been discussed earlier in Figures 5 and 6. bond in PEG detected by XPS and FTIR spectrums, which have been discussed earlier in Figures 5 According to Wang et al. [37], C=O bond can act as an electron injector that initiated the and 6. formation of hydroxyl radical, thus eventually degraded the MB dye pollutant. TP2 had undergone According to Wang et al. [37], C=O bond can act as an electron injector that initiated the the photoluminescence (PL) analysis low activation energy irradiation to observe the effect of C=O formation of hydroxyl radical, thus at eventually degraded the MB dye pollutant. TP2 had undergone activation under visible light irradiation. The PL emission spectra can be used to reveal the the photoluminescence (PL) analysis at low activation energy irradiation to observe the effect of efficiency C=O of charge carrier trapping, immigration transfer and to understand fate of activation under visible light irradiation.and The PL emission spectra can be usedthe to reveal thephoto-induced efficiency electrons and carrier holes in a semiconductor It is known the PL the spectrum is the result of the of charge trapping, immigration[52,53]. and transfer and to that understand fate of photo-induced electrons and holes inelectrons a semiconductor [52,53]. It the is known the PL spectrum the result of the recombination of excited and holes where lowerthat PL intensity means aislower recombination recombination of excited electrons and holes where the lower PL intensity means a lower rate of electron–hole pairs under light irradiation [54]. recombination rate of electron–hole pairs under light irradiation [54]. Figure 8 shows the PL spectra of pristine TiO2 , TP2 and unwashed samples of TP2 using the Figure 8 shows the PL spectra of pristine TiO2, TP2 and unwashed samples of TP2 using the excitation wavelength of 500 nm. The photoluminescence spectrum in this study that was conducted excitation wavelength of 500 nm. The photoluminescence spectrum in this study that was conducted under low excitation energy (500 nm) was meant to confirm that the excitation of electrons happened under low excitation energy (500 nm) was meant to confirm that the excitation of electrons under a low energy (visiblelevel light spectrum). It is shown increased PL intensity leads to happened under alevel low energy (visible light spectrum). It is that shown that increased PL intensity increased low excitation resulting higher photocatalytic activity [55]. It can be leads absorption to increasedofabsorption of lowenergy, excitation energy,inresulting in higher photocatalytic activity seen [55]. that It allcan samples an obvious PL signal withPL a similarly-shaped curve at thecurve wavelength be seenexhibited that all samples exhibited an obvious signal with a similarly-shaped at rangethe from 458 to 468 nm from with 458 the to TP2 sample giving the highest PL intensity followed by TP0 and wavelength range 468 nm with the TP2 sample giving the highest PL intensity followed TP0 and pristine 2. However, the PL energy is smaller the band gap of pristine TiO2 .byHowever, the PLTiO energy is smaller than the band gapthan energy of TiO According to 2 . energy TiO 2. According to Jing et al., 2006 they some of theoflower PL intensities of Jing et al., 2006 [56], they observed that[56], some of observed the lowerthat PL intensities semiconductor materials semiconductor materials are due to the that presence vacancy that acted as making an electron are due to the presence of oxygen vacancy actedofasoxygen an electron scavenger, thus electron scavenger, thus making electron recombination jump to the sub-band of TiO2. Moreover, the energy recombination jump to the sub-band of TiO2 . Moreover, the energy at the 458 to 468-nm wavelength at the 458 to 468-nm wavelength released from PL spectra is too high as compared to the excitation released from PL spectra is too high as compared to the excitation energy. This result is due to the energy. This result is due to the presence of the sensitizer, which allowed for the absorption of low presence of theenergy sensitizer, which forelectron-hole the absorption lowelectron excitation energy to occur and excitation to occur and allowed formed an pair.ofThis is then subsequently formed an electron-hole pair. This electron is then subsequently jumped to the conduction band of jumped to the conduction band of TiO2 and eventually recombined with the hole by releasing the TiO2 high and energy eventually recombined with the hole by releasing the high energy wavelength. wavelength.
0.8
PL Intensity (a.u)
TP2 0.6
TP2-Unwashed
0.4
0.2
Pristine TiO2
0 455
460
465
470
Wavelength (nm)
Figure 8. Photoluminescence spectra spectra of TiO 2, unwashed TP2 and TP2 samples. PL, Figure 8. Photoluminescence ofpristine pristine TiO TP2washed and washed TP2 samples. 2 , unwashed photoluminescence. PL, photoluminescence.
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As As can can be be seen seen in in Figure Figure 8, 8, all all TP2 TP2 samples samples (washed (washed and and unwashed) unwashed) have have shown shown higher higher PL PL intensity intensity as as compared compared with with pristine pristineTiO TiO22.. The intensity intensity from from the the unwashed unwashed TP2 TP2 sample sample might might be be due due to excitation energy. energy. to the the presence presence of of C–C C–C bond that allowed for the electron to recombine under low excitation The The TP2 TP2 sample sample (washed) (washed) has has recorded recorded the the highest highest PL PL intensity intensity as as compared compared to to others. others. Based Based on on the the XPS and FTIR spectra, this TP2 sample has a C=O bond as an extra species where it is not found XPS and FTIR spectra, this TP2 sample has a C=O extra species is not found in in pristine pristine and and unwashed unwashed TP2 TP2 samples. samples. Hence, it can be concluded concluded that the the highest highest intensity intensity of of TP2 TP2 showed showed aa significant significant presence presence of of C=O C=O bond, which acted as an electron injector in TP2 photocatalyst. The The electrons electrons that that were were injected injected to to the the conduction conduction band band of of TiO TiO22 created a series of chain reactions, which ·L−−11 MB MB dye dye at 75 min, as which help help to toachieve achieveaacomplete complete100% 100%decolorization decolorization of of 12 12mg mg·L as shown shown in in Figure Figure 7c. 2.8. 2.8. Recyclability Recyclability Study Study For stability study study of ofthe thephotocatalyst, photocatalyst,the thephotocatalytic photocatalyticactivity activity TP2 was carried For the the stability ofof TP2 was carried outout by −1 MB dye with 15-min intervals for 60 min in every −1 by eight cycles of photodegradation of 12 mg · L eight cycles of photodegradation of 12 mg·L MB dye with 15-min intervals for 60 min in every cycle. cycle. 9 shows the photodegradation of MB dyeTP2. using TP2.observed It was observed each FigureFigure 9 shows the photodegradation cycle ofcycle MB dye using It was that eachthat recycled recycled application produced 100% removal of MB; indicating a sustainable photocatalytic efficiency application produced 100% removal of MB; indicating a sustainable photocatalytic efficiency characteristic. In other other words, words, aa strong strong interaction interaction of of TiO TiO22 with with PEG PEG occurred characteristic. In occurred due due to to its its strong strong chemisorption on the surface of TiO , where it was not easily leached out, even through up to 2 chemisorption on the surface of TiO2, where it was not easily leached out, even through up to eight eight times times of of repeated repeated usage. usage.
Figure 9. 9. The recyclability graphs graphs of of TP2 TP2 under under the the photodegradation photodegradation of of MB MB dye. dye. Figure
2.9. Chemical Chemical Oxygen Oxygen Demand Demand Analysis Analysis 2.9. Decolorization of that there is complete removal of the carbons from Decolorization of dye dyedoes doesnot notmean mean that there is complete removal of organic the organic carbons the water samples. Mineralization, which is defined as the complete decomposition of organic from the water samples. Mineralization, which is defined as the complete decomposition of organic compoundsinto intoCO CO 2 and H2O, should be the target of any photocatalytic processes. One of the compounds 2 and H2 O, should be the target of any photocatalytic processes. One of the results results of mineralization is the lowering of the chemical oxygen demand (COD) of thesamples. treated of mineralization is the lowering of the chemical oxygen demand (COD) values ofvalues the treated samples. In this study, the presence of organic substances or intermediates can be detected by In this study, the presence of organic substances or intermediates can be detected by using a using COD a COD The test is attributed to the degradation dye, as by-products well as its by-products test. Thetest. COD testCOD is attributed to the degradation of MB dye,ofasMB well as its during the during the photocatalytic reaction theThere TP2 sample. There is a possiblefrom contamination from photocatalytic reaction using the TP2using sample. is a possible contamination DSAT, and these DSAT, and these contaminations were completely cleaned up during the washing process prior to contaminations were completely cleaned up during the washing process prior to the photodegradation theMB photodegradation MB dye. Figure 10 presents the detected values for of theMB mineralization of dye. Figure 10 of presents the detected COD values for the COD mineralization dye versus of MB dye versus irradiation time. The COD values (mg·L−1) detected were 0.81, 0.75, 0.60, 0.45, 0.30, 0.25, 0.10 and 0.05 and kept decreasing with time at 60, 120, 180, 240, 300, 360, 420 and 480 min,
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irradiation time. The COD values (mg·L−1 ) detected were 0.81, 0.75, 0.60, 0.45, 0.30, 0.25, 0.10 and 0.05 respectively. Hence, the complete mineralization of MB dye through the COD test was greatly Appl. Sci. 2016, 6, 451 with time at 60, 120, 180, 240, 300, 360, 420 and 480 min, respectively. Hence, 11 of 16 the and kept decreasing caused by the improved diffusion of dye into photocatalyst layers, stemming from the porous complete mineralization of MB dye through the COD test was greatly caused by the improved diffusion surface of the TP2Hence, photocatalyst sample. respectively. the complete mineralization of MB dye through the COD test was greatly of dye into photocatalyst layers, stemming from the porous surface of the TP2 photocatalyst sample. 0.8 COD Concentration (mg L-1)
COD Concentration (mg L-1)
caused by the improved diffusion of dye into photocatalyst layers, stemming from the porous surface of the TP2 photocatalyst sample.
0.6 0.4
0.8 0.6 0.4
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0 00
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200
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Figure 10. 10. The Thechemical chemicaloxygen oxygendemand demand(COD) (COD) analysis analysis of of TP2 TP2 under under photodegradation photodegradation of of MB MB dye. dye. Figure Figure 10. The chemical oxygen demand (COD) analysis of TP2 under photodegradation of MB dye.
3. Experimental Experimental Section 3. Section 3. Experimental Section 3.1. Preparation Preparation of Immobilized Immobilized TiO -PEG 22-PEG 3.1. of TiO 2-PEG 3.1. Preparation of Immobilized TiO The sample solution was prepared by by mixing 6.5 g6.5 of gtitanium dioxide (TiO(TiO P25P25 powder The sample solution was prepared bymixing mixing dioxide 2) Degussa 2 ) Degussa The sample solution was prepared 6.5 g of oftitanium titanium dioxide (TiO 2) Degussa P25 powder (20% rutile, 80% anatase) in 50 mL distilled water added with 1 mL of 8% (w/v) of (20% rutile, 80% anatase) in 50 mL distilled water added with 1 mL of 8% (w/v) of polyethylene glycol powder (20% rutile, 80% anatase) in 50 mL distilled water added with 1 mL of 8% (w/v) of polyethylene glycol Kenilworth, NJ,The USA, MW ==solution 6000). sample solution waswas sonicated (Merck, Kenilworth, NJ,(Merck, USA, MW = 6000). sample was sonicated under an ultrasonic polyethylene glycol (Merck, Kenilworth, NJ, USA, MW 6000).The The sample solution sonicated under an ultrasonic vibrator for 30 min to make it homogenized. The immobilized sample vibrator 30 min to make itforhomogenized. Theitimmobilized sample was preparedsample bywas using under anfor ultrasonic vibrator 30 min to make homogenized. The immobilized wasa prepared by using aapplied brush-coating method applied onto a clean glass plate prior to taping with brush-coating method onto a clean glass plate prior to taping with double-sided adhesive prepared by using a brush-coating method applied onto a clean glass plate prior to taping with double-sided adhesive tape (DSAT). The wet TiO2-PEG coated on glass was then dried using an tape (DSAT). adhesive The wet TiO coated glass was thencoated dried on using an was 850-W hotdried blower with a 2 -PEG double-sided tape Theon TiO 2-PEG glass then using 850-W hot blower with a (DSAT). temperature ofwet about 120 °C until dry. The process was continued by an ◦ temperature of about 120a C until dry. The process was°C continued byThe repeating thewas process of coating 850-W hot blower with about untilthe dry. process continued by repeating the process oftemperature coating ontoofdried TiO120 2-PEG until desired loading of immobilized onto dried TiO -PEG until the desired loading of immobilized TiO -PEG was achieved. Figure 2 achieved. 2 repeating the was process of coating onto dried 2-PEG the attached desired loading ofplate immobilized Figure 11 shows the TiO coated TiO2until on DSAT to the glass with 11 TiO2-PEG shows the coated TiO on DSAT attached to the glass plate with and without PEG binder. 2 and without PEG binder. was achieved. Figure 11 shows the coated TiO2 on DSAT attached to the glass plate with TiO2-PEG
and without PEG binder.
Figure 11. Picture of immobilized TiO2 with and without PEG binder and the molecular structure
Figure 11. Picture of immobilized TiO2 with and without PEG binder and the molecular structure of PEG. of PEG.
Figure 11. Picture of immobilized TiO2 with and without PEG binder and the molecular structure of PEG.
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3.2. Characterization Tests of Immobilized TiO2/PEG DSAT X-ray diffractionTests (XRD) spectra were using a Rigakuminiflex II, X-ray diffractometer 3.2. Characterization of Immobilized TiOobtained 2 /PEG DSAT (Rigaku, Tokyo, Japan). Structural information of the films was obtained in the range of 2θ angles X-ray diffraction (XRD) spectra were obtained using a Rigakuminiflex II, X-ray diffractometer from 3° to 80° with a step size increment of 1.00 s/step. FTIR spectra of powder samples were (Rigaku, Tokyo, Japan). Structural information of the films was obtained in the range of 2θ angles from recorded on Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device 3◦ to 80◦ with a step size increment of 1.00 s/step. FTIR spectra of powder samples were recorded on (Perkin Elmer, Waltham, MA, USA) with a diamond crystal. Spectra were collected in a frequency Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device (Perkin Elmer, range of 600 to 4000 cm−1 with 4 scans and a spectral resolution of 4 cm−1. The morphology of the Waltham, MA, USA) with a diamond crystal. Spectra were collected in a frequency range of 600 to samples −was observed with field-emitting scanning electron microscopy (FE-SEM, JSM-6700F, 4000 cm 1 with 4 scans and a spectral resolution of 4 cm−1 . The morphology of the samples was Akishima, Tokyo, Japan) with an accelerating voltage of 10 kV. The surface area of the immobilized observed with field-emitting scanning electron microscopy (FE-SEM, JSM-6700F, Akishima, Tokyo, TiO2 film powders was measured by nitrogen adsorption using the BET equation at 77 K Japan) with an accelerating voltage of 10 kV. The surface area of the immobilized TiO2 film powders (Micrometrics ASAP 2020M + C, Norcross, GA, USA). A UV–Vis spectrophotometer UV-2550, was measured by nitrogen adsorption using the BET equation at 77 K (Micrometrics ASAP 2020M + C, Shimadzu was used to obtain the UV–Vis reflectance spectrum of the powder sample. X-ray Norcross, GA, USA). A UV–Vis spectrophotometer UV-2550, Shimadzu was used to obtain the UV–Vis photoelectron spectroscopy (XPS) with a Thermo ESCALAB 250 spectrometer using a radiation reflectance spectrum of the powder sample. X-ray photoelectron spectroscopy (XPS) with a Thermo source of monochromatic Al Kα with the energy of 1486.6 eV, 200 W and a photoluminescence ESCALAB 250 spectrometer using a radiation source of monochromatic Al Kα with the energy analyzer (JovinYvon, Chiyoda-ku, Tokyo, Japan) was used to determine the photoluminescence of 1486.6 eV, 200 W and a photoluminescence analyzer (JovinYvon, Chiyoda-ku, Tokyo, Japan) was intensity of the samples. used to determine the photoluminescence intensity of the samples. 3.3. Washing Process of Immobilized Samples 3.3. Washing Process of Immobilized Samples The washing process was conducted to oxidize PEG and also to clean all unwanted The washing process was conducted to oxidize PEG and also to clean all unwanted contaminants contaminants from immobilized TiO2-PEG samples. The process was done by irradiating the from immobilized TiO2 -PEG samples. The process was done by irradiating the immobilized samples immobilized samples in distilled water inside a glass cell of 150 mm × 10 mm × 80 mm (length × in distilled water inside a glass cell of 150 mm × 10 mm × 80 mm (length × width × height). width × height). An aquarium pump model NS 7200 (Minjiang, Jiangmen, China) was used as an An aquarium pump model NS 7200 (Minjiang, Jiangmen, China) was used as an aeration source and aeration source and irradiated with a 55-W fluorescent lamp for 1 h. The washing process was irradiated with a 55-W fluorescent lamp for 1 h. The washing process was repeated once again by repeated once again by replacing the distilled water with a new amount of distilled water irradiated replacing the distilled water with a new amount of distilled water irradiated for another 30 min to for another 30 min to affirm that zero contamination is achieved. This contamination was measured affirm that zero contamination is achieved. This contamination was measured by using chemical by using chemical oxygen demand analysis (COD) to detect any presence of organic compounds in oxygen demand analysis (COD) to detect any presence of organic compounds in washed distilled washed distilled water. This process was done prior to the photodegradation of MB dye. water. This process was done prior to the photodegradation of MB dye. 3.4. 3.4.Photodegradation PhotodegradationofofMB MBDye Dye The activity ofofthe the catalyst tested bydegradation the degradation of methylene blue (MB), Fluka The activity catalyst waswas tested by the of methylene blue (MB), Fluka Analytical, Analytical, with a chemical formula: C 12H15O6; and the molecular structure of MB is shown in with a chemical formula: C12 H15 O6 ; and the molecular structure of MB is shown in Figure 12. Figure 12. The experimental wasmethod the same method from our previous report [57]. The The experimental procedure procedure was the same from our previous report [57]. The immobilized −1 MB dye placed inside a glass cell immobilized TiO 2-PEG was immersed into 20 − mL of 12 mg·L 1 TiO2 -PEG was immersed into 20 mL of 12 mg·L MB dye placed inside a glass cell under an aeration under anLight aeration Light was thenairradiated using a 55-W lamp,with Model Ecotone, source. wassource. then irradiated using 55-W fluorescent lamp,fluorescent Model Ecotone, visible light −2 of UV light detected as UV leakage. with visible light intensity measured for about 461 and 6.7 W·m − 2 intensity measured for about 461 and 6.7 W·m of UV light detected as UV leakage. A 4-mL aliquot Aof4-mL aliquot of treated MB dyeout wasfrom thenthe taken the glass cell atuntil 15-min intervals until by it treated MB dye was then taken glassout cellfrom at 15-min intervals it turned colorless turned colorless by measuringusing its concentration using UV spectrophotometer DRλ1900 measuring its concentration UV spectrophotometer Model HACH DR Model 1900 atHACH a 661-nm max at a 661-nm λ max detector (Hach, Loveland, CO, USA). The experimental procedure was repeated detector (Hach, Loveland, CO, USA). The experimental procedure was repeated by applying the same by applying the same steps for different andratios. different TiO2/PEG ratios. steps for different catalysts loading and catalysts different loading TiO /PEG 2
Figure12. 12.The Themolecular molecularstructure structurefor formethylene methyleneblue. blue. Figure
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3.5. COD Analysis Initially, the immobilized TiO2 /PEG DSAT (TP2) film was immersed in 20 mL of distilled water inside a glass cell under the irradiation of a 55-W compact fluorescent lamp. After 1 h of irradiation, the water sample was withdrawn and replaced with another set of distilled water using the same immobilized TiO2 /PEG film until 8 h of irradiation. The withdrawn water samples were then subjected to the COD test. It can be observed that MB dye and its generated by-products had undergone almost 100% complete mineralization after 8 h of irradiation using an immobilized TiO2 /PEG film. 3.6. Recyclability Study The recyclability study was carried out to see the effect of immobilized TiO2 -PEG towards photodegradation stability. The experiment was conducted initially through the photodegradation method. Immobilized TiO2 -PEG was then subjected to the washing process using distilled water and irradiated for 30 min. Both the photodegradation and washing procedures for the immobilized the TiO2 -PEG sample were then repeated until eight cycles. The photodegradation percentage of MB in every cycle was recorded at every 15-min interval until MB became colorless. 4. Conclusions An immobilized active TiO2 photocatalyst was successfully prepared via adding a small amount of PEG as a binder onto a support binder of double-sided adhesive tape (DSAT). It was observed that utilization of 10:0.1 of a TiO2 /PEG ratio at 0.3 g of catalyst loading produced an immobilized TiO2 with excellent photocatalytic activity. The preparation process did not produce any significant phase transformation, except for the typical TiO2 phase. From the XPS and FTIR spectra, both observed that washed TiO2 -PEG (TP2) produced C=O bond that was confirmed to initiate the photocatalytic activity of the sample and to be 1.8-times higher than pristine TiO2 under suspension mode in degrading 12 mg·L−1 MB dye. High PL intensity with low activation energy under immobilized TiO2 -PEG (TP2) proved that the presence of C=O increased the injected electron into the conduction band that eventually produced the hydroxyl radical agent used for the degradation of MB dye under visible light irradiation. Finally, TP2 or immobilized TiO2 -PEG was very stable and possessed excellent sustainable photocatalytic activity up to eight-times of reusability and comparable to recent photocatalysis cycles. As shown by the COD analysis, TP2 or immobilized TiO2 -PEG with DSAT leaves no organic pollutants during photodegradation cycles, which brings about a significant improvement in water quality. Acknowledgments: We would like to thank the Ministry of Education (MOE), Malaysia, for providing generous financial support under the Research Acculturation Grant Scheme (RAGS) grants (600-RMI/RAGS 5/3 (35/2014)) in conducting this study and Universiti Teknologi MARA (UiTM) for providing all of the needed facilities. Author Contributions: The experimental work and drafting of the manuscript were carried out by Raihan Zaharudin and assisted by Mohd Azlan Mohd Ishak, Khudzir Ismail and Ahmad Zuliahani participated in the interpretation of the scientific results and the preparation of the manuscript. Wan Izhan Nawawi supported the work and cooperation between UiTM Perlis and UiTM Shah Alam, supervised the experimental work, commented and approved the manuscript. The manuscript was written through comments and contributions of all authors. All authors have given approval to the final version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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